1. Field of the Invention
This invention relates to the bending of heat softened sheet material and in particular to horizontal press bending of heat softened glass sheets using a novel mold arrangement.
2a. Technical Considerations
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles and the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the window opening frames of the vehicles into which the windows are installed. It is also important that the windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with a clear viewing area.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat glass sheets to their softening temperature, shaping the heated glass sheets to a desired curvature, and cooling the bent glass sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace. The glass sheet is one of a series of sheets and is heated to its heat softening temperature and transferred into a shaping station adjacent to the furnace, for example, by a vacuum pick-up and shuttle arrangement. When in the shaping station, the glass sheet is pressed between a pair of vertically aligned upper and lower shaping molds. The upper mold is generally a vacuum mold that holds the heat softened glass sheet thereagainst by suction after shaping and the shaping surface of the lower mold generally complements the shaping surface of the upper mold. After shaping, a transfer and tempering ring having an outline shape conforming in elevational contour to that desired for the shaped glass sheet slightly inboard of its perimeter moves upstream into a position below the upper vacuum mold. Release of the vacuum deposits the glass sheet onto the tempering ring which supports the peripheral portion of the glass while it conveys the glass sheet into a cooling station for rapid cooling. The rapid cooling tempers the glass and increases its resistance to breakage.
As vehicle stylists strive towards more aerodynamic designs, the shapes of the glass sheets are becoming more complex. These designs include small radius bends, deep sags, and severe wraps along the longitudinal end portions of the glass. In order to press glass sheets with these configurations, the lower mold often times includes pivoting end sections to press the glass. Prior to pressing, the end sections are in a downward orientation so as to allow the heat softened glass sheet to be positioned as close as possible to the shaping surface of the lower mold. If these end portions of the lower mold were in an upward position prior to pressing, the glass sheet would have to be deposited onto the lower mold from an elevation at least equal to the depth of the wrap of the final glass configuration. Such an arrangement may cause misalignment or marking of the glass sheet due to the distance the glass sheet must be dropped onto the lower mold. After depositing the glass sheet on the lower mold, the end portions pivot upwardly as the glass sheet is pressed between the upper and lower molds.
It has been found that in using pivoting molds of this design, the pressed glass sheet may include optical defects along the line between the stationary center section of the lower pressing mold and the pivoting end portions. It would be advantageous to have a lower mold design that would reduce any optical defects resulting from the pressing operation while at the same time providing a pressing arrangement capable of fabricating the deep wrap and other complex glass designs.
2b. Patents of Interest
U.S. Pat. No. 4,229,200 to Seymour teaches a method of shaping glass sheets by drop forming. Auxiliary shaping means are employed in conjunction with a flat vacuum plenum in order to impart localized curvature to side portions of the glass sheet while the overall curvature is provided by the force of dropping the glass sheet onto a peripheral shaping mold. The flat plenum is adapted to engage less than the full area of the upper major surface of the glass sheet, leaving at least one side portion of the glass sheet extending beyond the edge of the plenum. The auxiliary shaping means act upon the extending portion of the glass sheet as the glass is held on the plenum, so as to preshape that portion of the glass sheet prior to dropping the sheet onto the shaping mold. Lifting means may be used for urging the extending portion of the glass sheet into engagement with the auxiliary shaping means. The lifting means may also be employed to prevent the extending portion of the glass sheet from sagging as the glass sheet is being lifted by the plenum.
U.S. Pat. Nos. 4,305,746 to Hagedorn et al and 4,496,386 to Hymore et al teach a method of and apparatus for press bending sheets into complex shapes. Glass sheets are preliminarily shaped by conveying the sheets over curved conveyor rolls. The apparatus further includes an outline-type press member having a first array of spaced apart shaping rail elements mounted in an array and adapted to pass upward between adjacent conveyor rolls to contact and support the under surface of the heat softened glass sheet. Another array of shaping rail extensions is positioned above the conveying rolls, mounted for movement into and out of association with the spaces between the spaced apart shaping rail elements of the first array of rail elements. The shaping rail elements press the perimeter of the glass sheet into engagement with an upper shaping rail or mold.
U.S. Pat. No. 4,501,603 to Frank et al teaches a method of and apparatus for shaping glass sheets to complicated shapes, including an end portion bent sharply relative to a main portion. The apparatus incorporates a vacuum holder and a movable shaping rail. The main portion of the hot glass sheet is held in vacuum engagement against the main portion of the shaping surface defined by the vacuum holder. The shaping rail engages the edge portion of the hot glass sheet to sandwich the latter between a corresponding end portion of the vacuum holder while vacuum is still applied to the vacuum holder.